Polysaccharide biopolymers are widely known and used water thickening agents, employed in a variety of commercial processes. One especially well-known process involves using these biopolymers as a thickening agent in a drive fluid employed in an enhanced oil recovery process.
It is well known that primary recovery of petroleum from subterranean deposits thereof can only recover from a small percent up to about 35% of the oil originally present in the formation. Waterflooding improves the total amount of oil recovered, but still leaves large amounts of petroleum in the formation. It is well known that waterflooding is not successful for recovering larger amounts of petroleum for a variety of reasons, one of which involves the poor displacement efficiency when waterflooding is applied to a subterranean oil formation. Since the viscosity of water is substantially less than the viscosity of the oil present in the formation, injection of water into a well and production of fluids from a spaced-apart well, while sweeping a sufficient portion of the reservoir to accomplish displacement of some petroleum from that reservoir, still bypasses large amounts of oil because of the tendency for water to move in a more or less direct path between the wells. It is also known that addition of viscosifying amounts of hydrophilic polymers to the injection water, resulting in a more favorable mobility ratio, accomplishes an increase in the amount of the reservoir swept by the injected fluid, and hence an increase in the amount of oil displaced by the injected fluid. Materials which have been used for this purpose in commercial operations and described in prior art publications include polyacrylamides, partially hydrolyzed polyacrylamides, co-polymers of acrylic acid and acrylamide, and polysaccharide biopolymers. Enhanced oil recovery processes using such viscous aqueous fluids, either alone or subsequent to injection of surfactant fluids, are well known in the art.
Biopolymers are favored for viscous flooding under certain conditions, as it is known that they are capable of producing relatively high fluid viscosities at low concentrations, they are resistant to shear degradation, and they maintain their viscosity in the presence of high salinities such as are frequently encountered in subterranean oil formations. It is also known that, with respect to the use of viscous fluids in combination with surfactant flooding, the biopolymers sometimes are more compatible with surfactant solutions than are some of the other hydrophilic polymers employed for this purpose.
When any fluid is injected into an injection well and fluids are recovered from a spaced-apart production well, it is known that the fluids follow a series of paths sometimes referred to as stream tubes, including the shortest path which is an essentially straight line between the injector and the producer, and a set of curved paths whose lengths increase as the deviation of these paths from the straight line increases. The rate of flow of fluids in the more remote paths decreases substantially as the distance from the straight line path increases, and it is known that in a typical situation encountered in oil recovery activities, the fluid flow rates can be 4 or 5 times lower in the outermost flow paths that in the central flow path. The lower the flow rate of fluid, the lower its shear rate. Many of the low concentration fluids containing hydrophilic polymers exhibit shear thinning i.e. the viscosity decreases as the shear rate increases. This should not be confused with shear degradation, which is a permanent alteration of the molecular structure of the polymer when the fluid is subjected to very high shear rates. Shear thinning simply means that the fluid traveling the longer flow paths, and hence moving at a lower velocity, will also exhibit a higher viscosity than the fluid in the more central, higher flow rate flow paths. This variation in the fluid viscosity between the central flowpath and more remote flowpaths, has a substantial detrimental effect on sweep efficiency.
In addition to the above-described phenomena, many petroleum formations are comprised of a plurality of layers with widely differing permeabilities, thus even the central flowpaths between injector and producer represent a plurality of flowpaths when viewed in a vertical plate through the injector and producer, and higher flow rates encountered in the high permeability layers cause higher shear rates, which decreases the viscosity of the hydrophilic polymer. Thus fluids flowing even in the central flowpaths between injector and producer, which are in lower permeability layers of a heterogeneous formation, exhibit higher viscosity than do the fluids in the high permeability, higher velocity layers.
The vertical and horizontal conformance of viscous fluids flowing through permeable formations in the above-described situations, will both be improved if the viscosity loss with increased shear rate of a fluid containing a hydrophilic polymer, is reduced.
In view of the foregoing discussion, it can be appreciated that there is a substantial need for an improved polymer which is less prone to shear thinning than the presently available commercial polymers.